The present invention relates to an optical fibre cable for telecommunication, in particular to a drop optical fibre cable suitably deployed both outdoor and indoor.
As “drop cable” it is meant an optical cable operating as the last link of an access distribution network, typically serving just one subscriber.
Optical fibre telecommunication cables for outdoor use should have mechanical characteristics such to withstand environmentally challenging situation. For example, as reported by GB2368404, aerial (or overhead) telecommunication drop cables are suspended from poles in a catenary and sag due to their own weight. The extent of the sag on installation is determined by the tension in the drop cable and is designed to be within a range of values determined by the acceptable drop cable tension and the acceptable extent of eventual sag to avoid hazard. In addition, externally mounted drop cables are subject to additional variable loading due to wind force and settling of moisture or ice formation. This additional loading results in strain in the drop cable and will affect all the components of the cable including the data carrying components.
Reinforcing elements are typically provided into the cable for increasing resistance to strain stress. For example, strength members, preferably two diametrically opposed ones, are provided embedded in the outermost sheath of the cable as shown, for example, in US 20100202741 and in U.S. Pat. No. 4,723,831. In the Applicants experience, the closer the strength members are to the surface of the cable the more efficient the load transfer is. The device securing the cable to the pole usually acts to increase the amount of radial compression on the cable, so as to increase the load transfer to the cable strength members as the load on the cable increases. If the radial thickness of the cable sheath over the strength member is too thin then the sheath material will fracture, but if the said thickness is too high then the amount of load being transferred to the strength member is reduced, as the cable sheath acts as a buffer and sheath shear and possibly break or slippage may occur.
The outer sheath of an aerial drop cable is typically made of a material having suitable mechanical and environmental resistance for outdoor use in the planned cable location. Example of materials apt to the scope are polyethylene, in particular high density polyethylene (HDPE) (see, for example, US2005002623), and polyvinylchloride (PVC) (see, for example, U.S. Pat. No. 4,723,831).
A multipurpose telecommunication drop cable, suitable both for outdoor (external, overhead, underground) and indoor use is desired.
According to some national standards, a cable containing no-low fire hazard materials can be installed within a building in limited length only. If the indoor length required for the connection is exceeded the cable must be changed to low fire hazard cable and this involves the installation of a transition joint. Alternatively the cable must be installed in some kind of containment. Both these solutions are expensive.
A cable having low fire hazard sheathing is not suitable for outdoor use where resistance to environmental (both mechanical and chemical) stresses is required, which may not sufficiently provided by the low fire hazard sheathing.
The already mentioned U.S. Pat. No. 4,723,831 relates to an optical fiber cable that can be used in the local distribution network, including in external plant applications such as distribution or buried service cable, and indoor applications such as riser or plenum cable. Said cable includes a first jacket, typically comprising a polyvinyl chloride material, a core member comprising at least one optical fiber, and a core wrap of woven glass fibers loosely surrounding the optical fiber(s). The cable further comprises three non-metallic strength members (or groups of strength members) completely embedded in the first jacket and coupled thereto. The voids between the optical fiber(s) and the core wrap are filled with a fire retardant grease composition.
Strain resistance is important for an indoor cable, too. During indoor deployment, the drop cable can be pulled for considerable length and through tortuous and also narrow passage. In addition, an indoor cable can suffer from temperature fluctuations, in particular from shrinkage at low temperatures after deployment. The presence of strength members is important for providing the cable with a suitable resistance to strain stresses. In addition, it is desirable to have strength members capable of bearing axial compression loads, such as those due to thermal shrinkage caused by temperature fluctuations.
Flextube™ cable of Draka described in the brochure Std FVDIFT-D2-KHKP-(2-4)-BBXS-0e of Jan. 15, 2010 comprises a micromodule housing optical fibres surrounded by an inner sheath of low smoke halogen free material. Said inner sheath, in turn, is enveloped by a dielectric reinforcement made of glass fibre reinforced plastic material and aramid yarns. An outer sheath made of HDPE embedding two diametrically oppose strength members surrounds the whole. The cable is intended for overhead installation on poles, installation in ducts or on front wall. Indoor installation of the cable is feasible after removal of the outer sheath. The presence of a dielectric reinforcement is said to provide said cable with resistance to strain stresses even when the outer sheath and the strength members embedded therein are removed.
Whilst aramid yarns as strength members are well proven, there may be a problem in using them when there is a defined maximum tensile strength requested for the cable. The load at which aramid yarns ultimately breaks is not as well defined as for metal or grp (glass reinforced polymer) strength members, but rather lays in a range which may provide uncertainty in the application, unless the size is increased, resulting in very high costs. In addition, aramid yarns would not provide resistance to axial compression, e.g. due to thermal shrinkage.
Strength members made of grp have a lower strength to diameter ratio than that of metallic strength member. As a consequence, if a cable endowed with grp members is desired to have the same tensile resistance than a cable with metallic strength members, such cable should have a greater outer diameter.
The Applicants faced the problem of providing a drop telecommunication cable suitable both for outdoor and to indoor use. In particular, the Applicants faced the problem of providing a cable with suitable strain and environmental resistance when used outdoor, and of retaining a sufficient strain resistance and fire performances when used indoor.
Strength members location close to the outer cable surface was believed to be required for transferring the load from the securing device to the strength members, but if said strength members are embedded in the outer sheath, made of a material suitable for outdoor application but not fire resistant, they get lost with the removal of the outer sheath at the moment of the indoor deployment.
The Applicant planned to design an optical fibre drop cable with strength members embedded into an inner, fire resistant sheath, while providing a suitable load transfer from the securing device to strength members so deeply embedded in the cable structure.